1. Field of the Invention
This invention relates to a sheet feeding apparatus which may be used for a printer, a facsimile apparatus or the like, and more particularly, to a sheet feeding device for feeding cut paper, rolled paper, an original, etc.
2. Description of the Related Art
FIG. 14 illustrates a schematic configuration of a conventional thermal recorder which uses rolled recording paper. In FIG. 14, thermal print head 1031 performs printing on recording paper. The recording paper is grasped and fed by platen roller 1032 and thermal print head 1031. Spring 1033 presses thermal print head 1031 against platen roller 1032. Reflection-type sensor 1034 detects the presence/absence of the recording paper. In addition, recording-paper guide roller 1035 and rolled thermosensitive recording paper 1036 are also shown.
In the above-described configuration, sensor 1034 detects only the presence/absence of recording paper, and does not detect the leading edge and the trailing edge of the recording paper. That is, sensor 1034 detects the presence of the recording paper when it is set, and detects the absence of the recording paper when the recording paper either is not set or has been used up.
Conventionally, in the printer unit of a facsimile apparatus or the like in which thermosensitive rolled paper or thermal transfer paper is used, oblique movement of the recording paper is one of the main causes of a paper jam. In some cases, the printing position shifts due to the oblique movement of the recording paper, thereby causing misprinting of necessary information.
In order to prevent the oblique movement of the recording paper, a sprawling sheet is inserted between respective sheets of rolled paper to provide parallelism with a feeding roller.
A conventional facsimile apparatus or a printer may be configured as illustrated in FIG. 15. Rolled thermosensitive recording paper, 1251, is fed by platen roller 1250, and is selectively heated by recording head 1252, to form an image. A sheet of recording paper 1251 on which the image has been formed is cut at the trailing edge of the image by cutters 1253a and 1253b, and the cut sheet is discharged.
In the above-described configuration, as shown in FIG. 16, end mark 1258 is printed on the back, opposite to the recording surface of recording paper 1251, over a range of about 1 m from the trailing edge of recording paper 1251 with a width of approximately 20-30 mm.
End-mark sensor 1256 is provided at a position facing the back of recording paper 1251 and corresponding to the position of end mark 1258 in the lateral direction. Accordingly, when a sufficient length of recording paper 1251 remains, end-mark sensor 1256 detects the white background of the recording paper 1251. When the length of the remaining portion of recording paper 1251 is approximately 1 m, end-mark sensor 1256 detects the black end mark 1258. Black plate 1257, shown in FIG. 15, is provided at a position facing end-mark sensor 1256, across recording paper 1251, so that the black plate is detected if recording paper 1251 is absent.
The facsimile apparatus performs a normal recording operation while end-mark sensor 1256 detects the white background of recording paper 1251. When end mark 1258 is detected during a recording operation, the recording operation is continued for the page being printed, and the subsequent recording operation is then stopped. In an apparatus having an intercepting function, the contents of pages after the recording operation is stopped are stored in a memory of the apparatus, and the stored recording is performed when new recording paper has been loaded. When end-mark sensor 1256 detects black at the start of the operation, the receiving operation is not performed at all, or is intercepted at the start of the operation.
When sheet-like recording paper is used, it is necessary to detect the position of the leading edge of the recording paper in order to determine the position to start recording. When the length of data to be recorded is shorter than the length of the recording paper, it is necessary to detect the trailing edge of the recording paper in order to discharge the recording paper after the completion of recording. When the length of data to be recorded is longer than the length of the recording paper, since the recording paper will be used up during the recording operation, it is necessary to prevent idle printing by interrupting recording after detecting the trailing edge of the recording paper.
In order to perform the above-described operations, it is necessary to provide sensors to precisely detect both the leading edge and the trailing edge of recording paper, in addition to providing a sensor for detecting the presence/absence of the recording paper, thereby causing an increase in the production cost.
The above-described method of using a sprawling sheet has the following problems:
(1) Oblique movement of recording paper cannot be prevented if parallelism is not provided between the sprawing sheet and the sheet-feeding roller (platen roller). Therefore, high accuracy in dimensions is required.
(2) It is difficult to set the recording paper.
In the above-described approach of using an end-mark sensor, since end-mark sensor 1256 detects both the state of the presence of end mark 1258 and the state of the absence of recording paper by the detection of the color black, it is impossible to discriminate between the two states.
Therefore, in the case of a facsimile apparatus, even if end mark 1258 is detected while receiving and recording an abnormally long page, platen roller 1250 continues to rotate when the amount of the contents to be recorded is longer than the remaining amount of recording paper, and part of the contents will not be recorded.
In addition, since the position of the end mark differs between manufacturers, the end mark cannot be detected when recording paper of a different manufacturer is used. Hence, it is impossible to know that the remaining amount of recording paper is small until the trailing edge of the recording paper passes. As a result, even if recording paper is suddenly used up during a recording operation, the platen roller continues to rotate until recording of the page being recorded is completed, and part of the contents will not be recorded.
Even if the contents to be recorded are not recorded on recording paper in the above-described manner, the transmitter will determine that the page having a missing portion has been completely transmitted. The page having the missing portion cannot be recovered even if intercepting is performed from the subsequent page.
When platen roller 1250 idly rotates, it directly contacts recording head 1252, thereby causing an increase in the load. Hence, the driving system for recording must be strengthened more than necessary.
When platen roller 1250 idly rotates, the trailing edge of the recording paper remains in a space between sheet guides 1255 and 1254. Since recording is completed in that state, and the recording paper is cut, a piece of the recording paper remains within the apparatus.